142 MASS. EXPERIMENT STATION BULLETIN 167. 



gations were based on the reaction of media which were adjusted by 

 either Schiiltz's^ or Fuller's^ scale, or the so-called "scale of reaction" 

 which will be discussed later. Considering these investigations in the 

 light of the physico-chemical methods of the present day, the influence 

 of the so-called "true acidity" or "true alkalinity" on bacteiial life is 

 not sufficiently known, because Kisch's method, as well as others which 

 will be presented in detail later, is not applicable. 



Recently, with the development of a physico-chemical method, it has 

 become possible to measure the hydrogen ion concentration and also to 

 apply it in the field of biologic investigation. Of late, as already related, 

 several biochemists and biologists have employed the method in various 

 lines of work, such as hydrogen ion concentration in blood, urine, wine, 

 milk, water, etc' As yet, references on the direct bacteriological work 

 are comparatively rare. Beside Biiinn's investigation, * which was men- 

 tioned in the introduction, Michaelis and Marcola ^ investigated the acid 

 production by B. coli, and found that it produces lactic acid in alkaline 

 lactose nutrient broth until it reaches a hydrogen ion concentration desig- 

 nated as Ph = 5, when the action ceases. Clark's recent investigations' 

 concerning the reaction of bacteriologic culture media, by both old (titri- 

 metric) and new (hydrogen electrode) methods, will be discussed later, 

 together with the physiologic importance of hydrogen ion concentration. 

 Clark pointed out the fallacies of the titrimetric method, and noted the 

 advisability of substituting a colorimetric method. He experimented 

 also with several different enzymes, such as trypsin, maltase, urease and 

 others, and marked the approximate range of hydrogen ion concentration 

 and degree of their activity. Clark and Lub^ employed the hydrogen 

 electrode to determine the change in hydrogen ion concentrations in 

 various cultures of the colon-serogenes family, and established the corre- 

 lation between the gas ratio and hydrogen ion concentration in the culture 

 medium. They recommend the use of indicators, i.e., paranitrqphenol 

 or methyl red, for diffei entiation of bacteria of this family. 



These investigations added some knowledge regarding the influence of 

 hydrogen ion concentration upon the bacterial life. This investigation 

 deals more comprehensively with the influence of hydrogen ion concentra- 

 tion on bacterial life, together with other matters. 



The proteolytic activity of bacteria has attracted much attention from 

 bacteriologists. Many observations and experiments have been carried 

 out since gelatin has been used as a culture medium. In 1886 Bitter* 

 discovered that microorganisms subjected to a temperature higher than 

 thermal death point retain the ability to liquefy gelatin, although the 



' Centralbl. f. Bakt., O, 1891, 10, 52. 



' Jour. Amer. Pub. Health Assn., 1895, 20, 381. 



' Michaelis, L. Die Wasserstoffionenkonzentration, 84, 1914, Berlin. 



* Uber das Desinfectionsvermogen der Sauren. Diss., Berlin, 1913. 

 ' Zeitschr. f. Immunitatsforschung, 1912, 14, 170. 



• Jour, of Infect. Diseases, V. 17, No. 1, July, 1915, 160-173. 

 ' Jour, of Infect. Diseases, V. 17, No. 1, July, 1915, 109-136. 

 « Archiv. f. Hygiene, Bd. 5, 1886. 



